This invention relates to a leather coating composition. More particularly, this invention relates to an aqueous leather coating composition containing a multi-stage emulsion polymer having a lower Tg (glass transition temperature) polymer stage which contains a copolymerized carboxylic acid and which has been contacted with a divalent metal oxide, hydroxide, or carbonate and a higher Tg polymer stage which has been prepared in the presence of 1% to 15% by weight based on the weight of that stage of chain transfer agent. Also, the invention relates to a method for coating leather with the composition of this invention.
The present invention serves to provide a protective coating that is aesthetically pleasing. The dried coating may be subsequently embossed with a desired imprint in a heated press. The softness of the coated leather and an effective embossed imprint are desirable aesthetic properties of the coated leather.
U.S. Pat. No. 5,723,182 discloses an aqueous leather coating composition containing a multi-stage emulsion polymer which has been contacted with a transition metal oxide, hydroxide, or carbonate. Further improvements in the softness of the coated leather and the ability of the coating to be embossed effectively are desirable
The problem faced by the inventors is the provision of an aqueous leather coating composition which yields a dried coated leather that has improved softness and embossability relative to prior compositions, preferably with little or no increase in surface tack of the dried coating. Surprisingly, the inventors found that the use of 1-15% by weight based on the weight of the second stage polymer of a chain transfer agent such as a mercaptan during the formation of the second (harder) stage of a multi-stage emulsion polymer reduced the stiffness and improved the embossability of leather coated with an aqueous composition containing the multi-stage polymer.
According to a first aspect of the present invention there is provided an aqueous leather coating composition including a multi-stage aqueous emulsion polymer including (i) a predominantly acrylic first stage polymer including at least one copolymerized monoethylenically-unsaturated nonionic monomer and from 0.5% to 10% of a copolymerized monoethylenically-unsaturated carboxylic acid monomer, based on the weight of the first stage polymer, the first stage polymer being substantially free from copolymerized multiethylenically-unsaturated monomer; the first stage polymer having a Tg less than 10xc2x0 C.; the first stage polymer having been contacted with a transition metal oxide, hydroxide, or carbonate at a pH of less than 9 in an amount greater than 0.1 equivalent of transition metal per equivalent of the copolymerized carboxylic acid monomer in the first stage polymer; and (ii) a second stage polymer including at least one copolymerized monoethylenically-unsaturated nonionic monomer and from 0% to 10% of a copolymerized monoethylenically-unsaturated carboxylic acid monomer, based on the weight of the second stage polymer, wherein the second stage polymer is formed in the presence of from 1% to 15% by weight based on the weight of the second stage polymer of chain transfer agent, provided that the second stage copolymerized carboxylic acid monomer is less than 25%, by weight, of the total copolymerized carboxylic acid monomer in the multi-stage emulsion polymer, the second stage polymer being substantially free from copolymerized multiethylenically-unsaturated monomer; the second stage polymer having a Tg greater than 20xc2x0 C.; and the second stage polymer being from 1% to 50% of the weight of the first stage polymer, based on dry polymer weights, and.
According to a second aspect of the present invention there is provided a method for coating leather including
(a) forming an aqueous leather coating composition including a multi-stage aqueous emulsion polymer formed by a method including
(i) forming a predominantly acrylic first stage polymer including at least one copolymerized monoethylenically-unsaturated nonionic monomer and from 0.5% to 10% of a copolymerized monoethylenically-unsaturated carboxylic acid monomer, based on the weight of the first stage polymer, the first stage polymer being substantially free from copolymerized multiethylenically-unsaturated monomer; and the first stage polymer having a Tg less than 10xc2x0 C.;
(ii) contacting the first stage polymer with a transition metal oxide, hydroxide, or carbonate at a pH of less than 9 in an amount greater than 0.1 equivalent of transition metal per equivalent of the copolymerized carboxylic acid monomer in the first stage polymer; and
(iii) forming a second stage polymer including at least one copolymerized monoethylenically-unsaturated nonionic monomer and from 0 to 10% of a copolymerized monoethylenically-unsaturated carboxylic acid monomer, based on the weight of the second stage polymer, wherein the second stage polymer is formed in the presence of from 1% to 15% by weight based on the weight of the second stage polymer of chain transfer agent, provided that the second stage copolymerized carboxylic acid monomer is less than 25%, by weight, of the total copolymerized carboxylic acid monomer in the multi-stage emulsion polymer, the second stage polymer being substantially free from copolymerized multiethylenically-unsaturated monomer; the second stage polymer having a Tg greater than 20xc2x0 C.; the second stage polymer being from 1% to 50% of the weight of the first stage polymer, based on dry polymer weights,
(b) applying the coating composition to leather; and
(c) drying the coating composition.
This invention relates to a leather coating composition and a method for coating leather including a multi-stage polymer prepared by emulsion polymerization. The multi-stage emulsion polymer contains a predominantly acrylic first stage polymer comprising at least one copolymerized monoethylenically-unsaturated nonionic monomer and from 0.5% to 10% of a copolymerized monoethylenically-unsaturated carboxylic acid monomer, based on the weight of the first stage polymer, the first stage polymer being substantially free from copolymerized multiethylenically-unsaturated monomer. By xe2x80x9cpredominantly acrylic first stage polymerxe2x80x9d is meant that greater than 50% of the copolymerized monomers forming the first stage polymer are acrylic, i.e., that they are selected from esters, nitrites, etc. of (meth)acrylic acid. By xe2x80x9cnonionic monomerxe2x80x9d herein is meant that the copolymerized monomer residue does not bear an ionic charge between pH=2-13. The first stage polymer contains at least one copolymerized monoethylenically-unsaturated nonionic monomer such as, for example, a (meth)acrylic ester monomer including methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, methyl methacrylate, butyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, acetoacetoxyethyl (meth)acrylate, acetoacetoxypropyl (meth)acrylate; styrene or substituted styrenes; butadiene; vinyl acetate or other vinyl esters; vinyl monomers such as vinyl chloride, vinylidene chloride, N-vinyl pyrolidone; and (meth)acrylonitrile. The use of the term xe2x80x9c(meth)xe2x80x9d followed by another term such as (meth)acrylate or (meth)acrylonitrile, as used throughout the disclosure, refers to both acrylate and methacrylate or acrylonitrile and methacrylonitrile, respectively.
The first stage polymer also contains from 0.5% to 10%, preferably from 1% to 5%, of a copolymerized monoethylenically-unsaturated carboxylic acid monomer, based on the weight of the first stage polymer, such as, for example, acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, monomethyl itaconate, monomethyl fumarate, monobutyl fumarate, and maleic anhydride.
The first stage polymer used in this invention is substantially free from copolymerized multiethylenically-unsaturated monomers such as, for example, allyl methacrylate, diallyl phthalate, 1,4-butylene glycol dimethacrylate, 1,2-ethylene glycol dimethacrylate, 1,6-hexanediol diacrylate, and divinyl benzene. By xe2x80x9csubstantially free from copolymerized multiethylenically-unsaturated monomersxe2x80x9d herein is meant that levels less than 0.1% based on the weight of the first stage polymer or the second stage polymer such as might be adventitiously introduced as impurities in monoethylenically-unsaturated monomers are not excluded from the polymer composition.
The glass transition temperature (xe2x80x9cTgxe2x80x9d) of the first stage polymer is less than 10xc2x0 C., as measured by differential scanning calorimetry (DSC) using the mid-point in the heat flow versus temperature transition as the Tg value. Chain transfer agents such as, for example, mercaptans may be used in an amount effective to provide lower molecular weights.
The first stage polymer is contacted with a transition metal oxide, hydroxide, or carbonate at pH less than pH=9, preferably at pH=3-6, in an amount greater than 0.1 equivalent of transition metal per equivalent of copolymerized carboxylic acid monomer in the first stage polymer according to the process disclosed in U.S. Pat. No. 5,221,284. The oxides, hydroxides, and carbonates of zinc, aluminum, tin, tungsten, and zirconium are preferred for low cost, low toxicity, and low color in the dried coating. Zinc oxide is more preferred. The transition metal oxide, hydroxide, or carbonate may be added slurried in water, optionally with an added dispersant such as, for example a low molecular weight homopolymer or copolymer of (meth)acrylic acid. The transition metal oxide, hydroxide, or carbonate may be added during the polymerization process or after the polymerization of one or more stages has been completed.
The multi-stage polymer also contains a second stage polymer comprising at least one copolymerized monoethylenically-unsaturated nonionic monomer and from 0% to 10% of a copolymerized monoethylenically-unsaturated carboxylic acid monomer, based on the weight of the second stage polymer, provided that the second stage copolymerized carboxylic acid monomer is less than 25%, by weight, of the total copolymerized carboxylic acid monomer in the multi-stage copolymer; the second stage polymer being substantially free from copolymerized multiethylenically-unsaturated monomer; the second stage polymer having a Tg of greater than 20xc2x0 C., and the second stage polymer being from 1% to 50%, preferably from 5% to 20%, of the weight of the first stage polymer, based on dry polymer weights. The copolymerized monoethylenically-unsaturated nonionic monomer, copolymerized monoethylenically-unsaturated carboxylic acid monomer, and copolymerized multiethylenically-unsaturated monomer of the second stage polymer are defined and exemplified as for the first stage polymer herein.
The second stage polymer must be formed in the presence of 1% to 15%, preferably from 2% to 5%, by weight based on the dry weight of the second stage polymer of a chain transfer agent, such as, for example, halogen compounds such as tetrabromomethane; allyl compounds; or mercaptans such as alkyl thioglycolates, alkyl mercaptoalkanoates, and C4-C22 linear or branched alkyl mercaptans. Chain transfer agent(s) may be added in one or more additions or continuously, linearly or not, over most or all of the entire reaction period or during limited portion(s) of the reaction period. The chain transfer agent is preferably a mercaptan, more preferably a mercaptan selected from methyl 3-mercaptopropionate, butyl 3-mercaptopropionate, n-dodecyl mercaptan, and mixtures thereof.
The polymerization techniques used to prepare such aqueous multi-stage emulsion-polymers are well known in the art such as, for example, U.S. Pat. Nos. 4,325,856; 4,654,397; 4,814,373; and 5,723,182. In the multi-stage polymerization process at least two stages differing in composition are formed in sequential fashion. In the emulsion polymerization process conventional surfactants may be used such as, for example, anionic and/or nonionic emulsifiers such as, for example, alkali metal or ammonium salts of alkyl, aryl, or alkylaryl sulfates, sulfonates or phosphates; alkyl sulfonic acids; sulfosuccinate salts; fatty acids; ethylenically unsaturated surfactant monomers; and ethoxylated alcohols or phenols. The amount of surfactant used is usually 0.1% to 6% by weight, based on the weight of monomer. Either thermal or redox initiation processes may be used. The reaction temperature is maintained at a temperature lower than 100xc2x0 C. throughout the course of the reaction. Preferred is a reaction temperature between 30xc2x0 C. and 95xc2x0 C., more preferably between 50xc2x0 C. and 90xc2x0 C. The monomer mixture may be added neat or as an emulsion in water. The monomer mixture may be added in one or more additions or continuously, linearly or not, over the reaction period, or combinations thereof
Conventional free radical initiators may be used such as, for example, hydrogen peroxide, sodium peroxide, potassium peroxide, t-butyl hydroperoxide, cumene hydroperoxide, ammonium and/or alkali metal persulfates, sodium perborate, perphosphoric acid and salts thereof, potassium permanganate, and ammonium or alkali metal salts of peroxydisulfuric acid, typically at a level of 0.01% to 3.0% by weight, based on the weight of total monomer. Redox systems using the above initiators coupled with a suitable reductant such as, for example, sodium sulfoxylate formaldehyde, ascorbic acid, isoascorbic acid, alkali metal and ammonium salts of sulfur-containing acids, such as sodium sulfite, bisulfite, thiosulfate, hydrosulfite, sulfide, hydrosulfide or dithionite, formadinesulfinic acid, hydroxymethanesulfonic acid, acetone bisulfite, amines such as ethanolamine, glycolic acid, glyoxylic acid hydrate, lactic acid, glyceric acid, malic acid, tartaric acid and salts of the preceding acids may be used. Redox reaction catalyzing metal salts of iron, copper, manganese, silver, platinum, vanadium, nickel, chromium, palladium, or cobalt may be used.
Such a multi-stage process usually results in the formation of at least two mutually incompatible polymer compositions, thereby resulting in the formation of at least two polymeric phases. The mutual incompatibility of two polymer compositions and the resultant multiphase structure of the polymer particles may be determined in various ways known in the art. The use of scanning electron microscopy using staining techniques to emphasize the difference between the appearance of the phases, for example, is such a technique.
The average particle diameter of the emulsion-polymerized polymer particles is preferred to be from 30 nanometers to 500 nanometers.
The aqueous leather coating composition is prepared by techniques which are well known in the coatings art. First, at least one pigment is well dispersed in an aqueous medium under high shear such as is afforded by a COWLES (R) mixer or, in the alternative, at least one predispersed colorant is used. Then the multi-stage emulsion-polymer is added under low shear stirring along with other coatings adjuvants as desired. The aqueous coating composition may contain, in addition to the pigment(s) and the multi-stage emulsion polymer, conventional coatings adjuvants such as, for example, emulsifiers, coalescing agents, curing agents, buffers, dullers, neutralizers, thickeners, rheology modifiers, humectants, wetting agents, biocides, plasticizers, antifoaming agents, colorants, waxes, and anti-oxidants.
The solids content of the aqueous coating composition may be from about 10% to about 50% by volume. The viscosity of the aqueous polymeric composition may be from about 50 centipoise to about 10,000 centipoise, as measured using a Brookfield viscometer; the viscosities appropriate for different application methods vary considerably.
The aqueous coating composition may be applied to leather such as, for example, mineral tanned or vegetable tanned leather including full-grain leather, buffed or corrected-grain leather, and split leather with or without a prior treatment with an impregnating resin mixture using conventional coatings application methods such as, for example, curtain coater and spraying methods such as, for example, air-atomized spray, air-assisted spray, airless spray, high volume low pressure spray, and air-assisted airless spray.
Abbreviations
MMA=methyl methacrylate
BA=butyl acrylate
EA=ethyl acrylate
AA=acrylic acid
50% relative humidity. The samples were clamped at a grip distance of 2.54 cm (1 inch) and pulled at a crosshead speed of 25.4 cm(10 inches)/ minute. Stiffness was measured as the psi at 100% elongation (M100 ). Stiffness is considered significant only on a relative basis within a single series to eliminate variations in temperature, humidity, etc. from day to day.
The following examples are presented to illustrate the invention and the results obtained by the experimental methods, unless otherwise noted.